1. Field of the Invention
The present invention concerns a radio-frequency antenna arrangement for acquisition of a magnetic resonance signal.
2. Description of the Prior Art
Magnetic resonance signals are very weak signals. A good signal-to-noise ratio (SNR) is therefore of decisive importance for a qualitatively high-grade evaluation of the magnetic resonance signals acquired by a radio-frequency antenna in a magnetic resonance system. In the prior art significant efforts are made in order to optimize the SNR.
Radio-frequency antennas for magnetic resonance signals are always fashioned in the prior art as antennas that acquire the magnetic component of the electromagnetic signals that arise due to magnetic resonance. They are tuned to a resonance frequency by capacitors. The resonance frequency corresponds in the ideal case to the Larmor frequency of the magnetic resonance system. The radio-frequency antennas are fashioned, for example, as a single winding or as a group of windings, as a saddle coil, as a birdcage resonator, etc. In magnetic resonance systems with a basic magnetic field up to approximately 1.5 T, good SNR can be achieved for most purposes with such radio-frequency antennas. In practice the actual achieved SNR lies relatively close to the theoretically possible SNR.
More recently, magnetic resonance systems have been developed with higher basic magnetic fields, for example with basic magnetic fields of 3 T, 5T or even more. In these magnetic resonance systems it has surprisingly occurred that the SNR achieved in practice always lags further behind the theoretical optimum. The SNR therefore must be improved in order to enable a qualitatively high-grade evaluation of the magnetic resonance signal.
A dipole antenna that is divided into two dipole halves is known from WO 03/044892 A1. The dipole halves are connected with one another by an inductor. A signal acquired from the dipole antenna is tapped via the inductor.
A radio-frequency antenna arrangement for acquisition of a magnetic resonance signal is known from EP 1 624 314 A1. The radio-frequency antenna has two electrodes that are connected with one another by a discrete tuning device. The radio-frequency antenna is tuned to a resonance frequency by the tuning device. The tuning device has a tuning element that influences the resonance frequency of the radio-frequency antenna, this tuning element functioning as an inductor. The radio-frequency antenna furthermore has two tap points at which an acquisition signal representative of the acquired magnetic resonance signal is tapped and conducted away via feed lines. One of the tap points is arranged between each of the electrodes and the tuning element.